1. Technical Field
The present invention relates generally to fuel control systems and, more particularly, to a method of determining the percent alcohol content of a fuel used in a motor vehicle capable of operating on more than one type of fuel.
2. Discussion
Environmental and energy independence concerns have stimulated the development of alternative transportation fuels, such as alcohol fuels, for use in automobiles. Alcohol fuels include methanol and ethanol. A flexible fueled vehicle capable of operating on gasoline, or alcohol fuel, or any mixture of the two fuels, is therefore in demand. Modifications to the engine are necessary when operating on different fuels because of the different characteristics of each fuel. For example, an engine operating on ethanol or E85 (a blend of 85% ethanol and 15% gasoline) requires approximately 1.4 times the amount of fuel relative to gasoline at stoichiometry due to a lower energy content of the ethanol.
Air/fuel ratio in internal combustion engine design is typically considered to be the ratio of mass flow rate of air to mass flow rate of fuel inducted by an internal combustion engine to achieve conversion of the fuel into completely oxidized products. The chemically correct ratio corresponding to complete oxidation of the products is called stoichiometric. If the air/fuel ratio is less than stoichiometric, an engine is said to be operating rich, i.e., too much fuel is being burned in proportion to the amount of air to achieve perfect combustion. Likewise, if the air/fuel ratio is greater than stoichiometric, an engine is said to be operating lean, i.e., too much air is being burned in proportion to the amount of fuel to achieve perfect combustion. Alcohol fuels have a lower air/fuel ratio than gasoline at stoichiometric, so that the engine must be compensated for in the rich direction as the percentage of alcohol in the fuel increases.
For example, U.S. Pat. No. 5,119,671, entitled "Method for Flexible Fuel Control", hereby expressly incorporated by reference, discloses a fuel composition sensor that measures the percent alcohol content of the fuel and relays that information to an engine controller or the like so that dependant variables such as spark timing and air/fuel ratio can be adjusted accordingly. Also, U.S. Pat. No. 5,255,661, entitled "Method for Determining Fuel Composition Using Oxygen Sensor Feedback Control", hereby expressly incorporated by reference, provides a method for determining the percent alcohol content of the fuel in the fuel tank utilizing an oxygen sensor feedback control loop to sense changes in air/fuel ratio and relay that information to the engine controller so that dependant variables can be adjusted accordingly.
Although these methods work well, there is room for improvement in the art. For instance, according to the prior art, there is no clear separation between gasoline operation and alcohol operation. Also, the prior art does not provide a way to insure that all of the alcohol is absent from the fuel before the on board diagnostics monitors (OBDII) are reenabled. Additionally, the prior art does not provide a way to identify users of high alcohol concentration fuel so that engine parameters can be modified accordingly. Moreover, the prior art does not provide a way for the fuel system monitor to detect errors during alcohol operation.